Methods of preparing thallium system superconductors are documented in the literature. One method of preparation is reported in "Bulk Superconductivity at 120K in the Tl--Ca--Ba--Cu--O System," Z. Z. Sheng and A. M. Hermann, Nature Vol. 332, Mar. 10, 1988, pp. 138-139. Briefly described, appropriate amounts of powdered Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 were mixed, ground, and pressed into pellets. The pellets were heated in a furnace at 880.degree.-910.degree. C. with an oxygen flowing atmosphere for three to five minutes. After heating, the pellets were quenched to room temperature in air or furnace cooled to room temperature. Basically, the pellets were reaction sintered by the heat treatment, forming a superconducting conductor.
Thin films of thallium superconductors have also been made by sequential thermal evaporation, sequential electron beam evaporation, and spray pyrolysis of nitrate solutions containing the precursor cations, such as calcium, copper, and barium. A limitation for these methods is the time needed to deposit a layer about 10-100 .mu.m of the precursor deposit on the substrate.
The above-mentioned sintered ceramic pellets, thin films, and spray pyrolysis nitrate solutions are subsequently treated by heating in flowing air or oxygen at temperatures of about 800.degree.-900.degree. C. to form the superconducting compositions. Thus, another drawback for these systems is that thallium oxide has an appreciable vapor pressure at the temperature required to form the superconductor. As a result, thallium can be vaporized during the 800.degree.-900.degree. C. annealing temperatures leading to the loss of thallium from the superconductor.
Still a further drawback with the above methods of making thallium superconductors is that low current carrying capacity occurs when the grains are weakly linked or poorly connected. To achieve high current carrying capacity in these superconductors, the microstructure of the superconductor needs to have strongly interconnected or linked grains. However, this type of microstructure is not formed uniformly and consistently using present methods of fabrication.
Thus, there is a need for a method to fabricate thallium-containing superconductors where the method produces a uniform superconducting phase and microstructure over long lengths of superconductor material. There is also a need for a method that can process the thallium superconductor at high temperatures in the presence of an appropriate partial pressure of a reactive thallium vapor. There is an additional need to have a method of fabricating superconducting conductors rapidly and economically.